This invention relates to filament wound pressure vessels and, more particularly, to a filament wound hydropneumatic accumulator tank containing a flexible diaphragm which separates the interior of the tank into a compressible gas containing chamber and a liquid containing chamber.
Accumulators or pressure control devices according to this invention are especially adapted for liquid dispensing systems and particularly for water systems dependent upon well water, a pump and, because of intermittent use, an intermittent source of pressurization for the system. A simple prior art system for accommodating the pressurization of a well water system, for example, in a dwelling, comprises a pipe extending into the underground water supply, a pump and a reservoir tank. Frequently, the tank is a metal container having no valve and in which air is merely trapped. In some installations, a valve, such as a conventional tire valve, may be positioned at or near the top of the container and an outlet may be provided near the bottom. In many cases, a diaphragm is not provided but it is highly desirable to separate the air from the water in the tank since air will dissolve in the water and may create a "white water" problem at the taps. A charge of air is introduced in the tank, usually occupying about one half of the volume of the tank and to a pressure sufficient to force water into the plumbing system connected thereto. A pressure switch senses the pressure within the tank or in the water line, as the case may be, and operates between limits, for example, between 20 and 40 psi. If the pressure falls below 20 psi the pump is activated and continues to operate until the pressure in the system is built up to 40 psi, causing the pump to be turned off by the pressure switch.
Typical prior art accumulators include a pressure vessel having an expansible bag therein which may be either an inflatable bag which stretches upon the introduction of air or which merely unfolds as the air volume increases or the expansible bag may unfold and also expand. An air valve extends through one end of the tank and an inlet and outlet aperture is provided at the other end of the tank for fluid communication with the water system. As water is pumped into the tank, the bag is forced upwardly by the incoming water. If the bag is the type which merely unfolds, this upward movement is characterized by a sharp annular crease adjacent the inner sidewall of the vessel, which progresses upwardly as the water rises and downwardly as the water is forced from the tank by the pressure in the bag. Thus, there is a substantially constant flexing of major portions of the sidewall of the bag which causes eventual failure, particularly if sand or silt finds its way into the water system due to inadequate or non-existent strainers in the well. Accumulator bags which stretch upon inflation are subject to fatigue and eventual failure particularly if the water is acidic or alkaline since stretching tends to open the pores of the bag material to chemical attack.
Separator bags or diaphragms in pressure tanks are either diaphragm type separators peripherally sealed to the sidewall of the tank usually at an assembly seam wherein the tank is formed by a pair of cup shaped halves or comprises a removable cell which may be removed and replaced upon failure. Both arrangements have advantages and disadvantages. The primary advantage of a diaphragm type separator is that the diaphragm may be constructed from a relatively heavy gauge, plastic or desirably butyl rubber and may be shaped to conform to the cross-section of the tank to eliminate stretching. This arrangement, however, involves the dual problem of providing a pressuretight seal between the mating halves of the pressure vessel and between the sidewall of the vessel and the diaphragm. For the sake of economy attempts have been made to combine the seal between the tank halves and the seal between the diaphragm and the sidewall in a single assembly. This arrangement, however, has not been entirely successful and tank leakage has resulted. Furthermore, these arrangements usually involve protruding flanges and clamps on the exterior of the tank which interfere with attempts to helically wind the tank for added reinforcement.
In order to eliminate the problem of adequate seals, a seamless tank having a removable and replaceable accumulator bag has been developed. Such an arrangement is shown in U.S. Pat. No. 4,214,611. Although the arrangement shown in that patent eliminates the problems involved in joining and sealing tank sections to each other and to a diaphragm, the bag must be fabricated from a relatively thin material so that the bag may be inserted through a relatively small axis opening during assembly. While the arrangement performed its intended function, it was found that the frequency of bag failure required excessive servicing. Moreover, since the bag was assembled in the tank in a blind fashion the interior of the tank could not be adequately inspected for foreign objects or rough interior surfaces which tend to lacerate the bag in use.